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The U.S. federal government has widely claimed that the potency levels of marijuana have risen anywhere from 10 to 25 times since the 1960s though these claims lack substance.

While testifying in front of the U.S. House Subcommittee on Crime, Director of the National Institute on Drug Abuse Alan Leshner said in 1999, “There’s no question that marijuana, today, is more potent than the marijuana in the 1960s. However, if you were to look at the average marijuana potency which is about 3.5 percent, it’s been relatively stable for the last 20 years. Having said that, it’s very important that what we have now is a wider range of potencies available than we had in the 1970s, in particular.”

However, those supporting legalization of marijuana are of the view that the data is skewed as testing was only conducted on marijuana of specific geographic origins in the 1960s and 1970s and it cannot be believed to be a representative of marijuana potency overall.
It is worthwhile to note that a type of Mexican marijuana contains low levels of THC — 0.4 to 1 percent while typical THC levels range from 0.3 to 4 percent though THC levels are as high as 15 percent in some plants. The potency of marijuana is dependent on many factors, including growing climate and conditions, plant genetics, harvesting and processing. Moreover, female plant varieties of marijuana tend to have higher levels of THC than male varieties.

In order to determine the average potency levels of marijuana, researchers need to evaluate a cross section of cannabis plants that was not done in the 1960s and 1970s.